A semiconductor memory is an indispensable component of various electronic device systems, and a nonvolatile semiconductor memory has a characteristic of retaining data even if its power is off, thus it is widely used in various mobile, portable apparatuses, such as cell phone, laptop, palmtop, or the like. With an increasing shrinkage of feature size, a conventional floating gate structure is gradually approaching the bottleneck, however, emerge of a new type of resistive random access memory (RRAM) brings hopes for fabricating a smaller, rapider, more economic nonvolatile memory. The RRAM has vantages of easy fabricating process, rapid read/write speed, high storage density, nonvolatility and excellently compatibility with a conventional silicon integrated circuit process, and thus has huge potential in application.
A RRAM is a nonvolatile memory based on electrically induced resistive change effect of some materials. The RRAM has a simple Metal-Insulator-Metal (MIM) capacitor structure as a functional device, in which an insulating layer material has a characteristic of electrically induced resistive change and has a resistive which may be reversibly changed under a specific external electrical signal. A process in which a resistive of the RRAM transits from a high resistive state (OFF-state) to a low resistive state (ON-state) is referred as a program (set) operation; a process in which the resistive of the RRAM transits from a low resistive state to a high resistive state is referred as an erase (reset) operation. The RRAM is mainly classified into unipolar type and bipolar type, depending on whether the voltage polarities occurred in Set/Reset processes are identical or not. The unipolar type RRAM has identical polarities in the Set/Reset process, while the bipolar type RRAM has different polarities in the Set/Reset process.
One transistor one RRAM (1T1R) is a currently typical RRAM structure, i.e. one RRAM and a drain terminal of one transistor (MOSFET) as a switch are serially connected. The transistor is referred as a selection transistor. The selection transistor in conventional 1T1R is formed on a bulk silicon substrate, and a RRAM in the conventional 1T1R is formed over the selection transistor with a thick oxide isolation layer therebetween. Since Set/Reset currents required in a RRAM for programming is relatively large, it is necessary to design the selection transistor MOSFET with a large width/length ratio in order to provide a sufficiently large saturation area current, which is adverse to the integration of memory with high density.